![]() Q – perfusion – the blood that reaches the alveoli via the capillaries.V – ventilation – the air that reaches the alveoli.Time constant is proportional to by resistance and compliance.In respiratory physiology, the ventilation/perfusion ratio ( V/Q ratio) is a ratio used to assess the efficiency and adequacy of the matching of two variables: the time required to inflate the lung unit if the flow rate is constant throughout.the time required to inflate to 63% of final volume if inflation is prolonged indefinitely.Time constant Definition of time constant NB: Normal tidal breathing is about 0.5L/sec => ventilation of apex:base = 1:1 or 1+:1 fast inspiration (flow rate > 1.5 L/sec) from FRC.during slow inspiration from residual volume.In anaesthetized patients, better ventilation in upper side. ![]() Alveoli on dependent side is less expanded.Diaphragm on the lower side is higher and thus contract better.Right lung ventilates better than left, because of its larger size.īetter ventilation on dependent side because: Right vs left lung Supine or erect position Thus, lung base is only ventilated intermittently in emphysema. => during inspiration, intrapleural pressure at lung base take a while to become negative and airflow occurs => at base, intrapleural pressure may even become positive ![]() => intrapleural pressure becomes less negative Where the elastic recoil of lung is reduced (e.g. Only the first part of inspired gas goes preferentially to apex. => initially, apex will expand first before intrapleural pressure at base becomes negative and air flows into base. Lung volume and recoil Very low lung volumeĪt very low lung volume (lower than closing capacity),īecause the lung base is so compressed that intrapleural pressure becomes positive,īecause small airways at base are closed, compliance at base is more optimal and is higher than apex. ![]()
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